1. Field of the Invention
The present invention relates to a lithographic apparatus and a method for manufacturing a device.
2. Description of the Related Art
A lithographic apparatus is a machine that applies a desired pattern onto a substrate, usually onto a target portion of the substrate. A lithographic apparatus can be used, for example, in the manufacture of integrated circuits (ICs). In such a case, a patterning device, which is alternatively referred to as a mask or a reticle, may be used to generate a circuit pattern to be formed on an individual layer of the IC. This pattern can be transferred onto a target portion (e.g. including part of, one, or several dies) on a substrate (e.g. a silicon wafer). Transfer of the pattern is typically via imaging onto a layer of radiation-sensitive material (resist) provided on the substrate. In general, a single substrate will contain a network of adjacent target portions that are successively patterned. Conventional lithographic apparatus include so-called steppers, in which each target portion is irradiated by exposing an entire pattern onto the target portion at once, and so-called scanners, in which each target portion is irradiated by scanning the pattern through a radiation beam in a given direction (the “scanning”-direction) while synchronously scanning the substrate parallel or anti-parallel to this direction. It is also possible to transfer the pattern from the patterning device to the substrate by imprinting the pattern onto the substrate.
One of the most challenging requirements for micro-lithography for the production of integrated circuits as well as liquid crystal display panels is the positioning of stages. For example, sub-100 nm lithography demands substrate- and mask-positioning stages with dynamic accuracy and matching between machines to the order of 1 nm in all 6 degrees of freedom (DOF).
A popular approach to such demanding positioning requirements is to sub-divide the stage positioning architecture into a coarse positioning module (e.g. an X-Y table or a gantry table) with micrometer accuracies but travelling over the entire working range, onto which is cascaded a fine positioning module. The latter is responsible for correcting for the residual error of the coarse positioning module to the last few nanometers, but only needs to accommodate a very limited range of travel. Commonly used actuators for such nano-positioning include piezoelectric actuators or voice-coil type electromagnetic actuators. While positioning in the fine module is usually effected in all 6 DOF, large-range motions are rarely required for more than 2 DOF, thus easing the design of the coarse module considerably.
The micrometer accuracy desired for the coarse positioning can be readily achieved using relatively simple position sensors, such as optical or magnetic incremental encoders. These can be single-axis devices with measurement in one DOF, or more recently multiple (up to 3) DOF devices such as those described by Schäffel et al “Integrated electro-dynamic multicoordinate drives”, Proc. ASPE Annual Meeting, California, USA, 1996, p. 456–461. Similar encoders are also available commercially, e.g. position measurement system Type PP281R manufactured by Dr. J. Heidenhain GmbH. Although such sensors can provide sub-micrometer level resolution without difficulty, absolute accuracy and in particular thermal stability over long travel ranges are not readily achievable.
Position measurement for the mask and substrate stages at the end of the fine positioning module, on the other hand, has to be performed in all 6 DOF to sub-nanometer resolution, with nanometer accuracy and stability. This is commonly achieved using multi-axis interferometers to measure displacements in all 6 DOF, with redundant axes for additional calibration functions (e.g. calibrations of interferometer mirror flatness on the substrate stage).
With the above approach, every time the stage is brought (back) into the range of the fine positioning module, the position of the stage has to be (re)calibrated in six degrees of freedom. This may take a considerable amount of time, and as a result the throughput of the lithographic apparatus may be decreased.
Furthermore, with the above approach, in the case of a lithographic apparatus including two substrate stages, one of the substrate stages can eclipse the substrate stage for a signal of one of the interferometers; the first substrate stage can be located between the interferometer and the second substrate stage.
U.S. Pat. No. 6,785,005 (herein incorporated by reference) discloses such position measurement system having two substrate stages. In this system the eclipse problem as indicated above is solved by providing the substrate stage which is located the furthest from the interferometer, with a mirror surface which is larger than the first substrate stage, so that in every position of the first substrate stage a signal of one of the interferometers can be directed on the mirror surface of the second substrate stage. However, this solution is unsatisfactory since the substrate stage have different sizes. Furthermore, the solution makes a complex switching between the interferometers necessary when the first substrate stage moves before the second substrate stage.
U.S. Pat. No. 6,879,382 (herein incorporated by reference) discloses another position measurement system for continuously measuring the position of two substrate stages of a lithographic apparatus. The position measurement system includes interferometers to determine the position of both substrate stages. Although the position measurement system of U.S. Pat. No. 6,879,382 makes determination of the position of the two substrate stages in the x-y plane possible, it does not offer the flexibility which is desired to make an immersion head take-over possible as described in the co-pending application having U.S. application Ser. No. 11/135,655 the contents of which is herein incorporated by reference.